The present invention relates to a method for hot dip galvanizing metallic workpieces by dipping them into a zinc melt, the workpieces being subjected to a pretreatment for cleaning their surfaces and coating them with an intermediate layer which assures a reaction with the zinc melt over the entire surface of the workpieces and wherein the workpieces are dipped into the zinc melt with their surfaces in a dry state and are removed from the zinc melt after a predetermined period of time. Such a method is disclosed in East German Patent No. 124,923.
Metallic workpieces as used in the present invention are workpieces of steel or iron materials which can be protected against corrosion by immersing them into a zinc melt. Depending on their size, the workpieces are immersed into the zinc melt either individually or simultaneously in larger quantities. In the prior art hot dip galvanizing process, the workpieces are pretreated on their surfaces and then coated with the desired zinc layer by immersing them into a zinc melt. Conventional zinc melts essentially comprise zinc and generally about 1% lead as well as metals, such as aluminum, iron, cadmium, copper and tin, as alloying elements or impurities, respectively.
Before being immersed into a hot dip galvanizin vessel, the workpieces must be pretreated in such a manner that their surfaces can be wet everywhere by the molten zinc. Such a pretreatment is the only way to assure that the zinc melt can react uniformly with the surfaces of the workpieces to form a complete, uninterrupted coating thereon.
The pretreatment of the workpiece surfaces can be performed in various ways. If steel strip is to be hot dip galvanized in a continuous passage, the pretreatment is usually a heat treatment process as disclosed, for example, in German Offenlegungsschrift No. 2,537,298 and corresponding U.S. Pat. No. 3,936,543, and in British Patent No. 1,496,398 and corresponding U.S. Pat. No. 3,925,579. In such a process it is necessary for the bright-annealed steel surface formed by the heat treatment to not come into contact with air before being immersed into the zinc melt so that the steel surface remains free of oxides. When hot dip galvanizing individual metallic workpieces, such thermal pretreatment is hardly feasible because of the apparatus involved. Therefore, such workpieces are usually pretreated in aqueous solutions, and less frequently by mechanical means, i.e. blasting.
Generally, the workpieces must first be degreased and in this way made wettable by water. Alkaline degreasing and cleaning solutions are customary for this purpose. After degreasing, the workpieces are rinsed in water. Thereafter, they are dipped into a pickling bath and after pickling they are rinsed again. To simplify the process, it is also possible in certain cases, to perform a so-called combined pickling/degreasing step, in which case the separate degreasing and rinsing can be omitted. Pickling is effected, for example, in diluted hydrochloric acid or in diluted sulfuric acid.
If the workpieces are to be wet-galvanized in a zinc melt, they are usually first immersed in acid and then sent wet through a flux coating, which is floating on the zinc melt, into the liquid zinc. See East German Patent No. 124,923. If, however, the so-called dry galvanizing process is employed, the workpieces are immersed in a solution of a fluxing agent and then dried, so that the workpiece surface is coated with a layer of fluxing agent. Only then are the workpieces dipped into the liquid zinc melt. To obtain a thinner zinc layer during the galvanizing process, and thus save zinc, East German Patent No. 124,923 proposes to precipitate copper on the steel surface before or during the immersion of the workpieces in the fluxing agent solution. This additionally applied copper layer is intended only to reduce the thickness of the zinc layer. The use of a fluxing agent cannot be left out.
The reaction of the fluxing agent with the workpiece surface during the immersion into the zinc melt produces a violent pickling effect which is considered necessary in hot dip galvanizing processes to obtain a uniform and complete zinc coating. This reaction results in a heavy emission of pollutants, such as, for example, ammonia, hydrochloric acid, ammonium chloride, zinc oxide and zinc chloride. Moreover, the immersion of the workpieces into the zinc melt produces large quantities of zinc ash and scrapings on the surface of the zinc melt which must be removed by skimming before the workpieces are pulled out of the zinc melt. This causes great losses of zinc. Moreover, the contaminants which ascend in the smoke leaving the zinc melt have a considerable impact on the environment. It is therefore necessary to collect the smoke and remove the contaminants by purification so that the exhaust gas can be made harmless. The removal of such contaminants, for example with the aid of gas purification systems, requires a large amount of apparatus.
Due to the poor conditions for emission purification, attempts have been made for years to develop and use low-smoke fluxing agents so as to reduce the contaminant content of the exhaust gas. In these attempts, different fluxing agents were employed, i.e., for example, solutions of different salts. Such salts are generally more expensive than the classical salts, zinc chloride and ammonium chloride, so that the costs are higher. Moreover, the use of low-smoke fluxing agents still involves a considerable amount of contaminant emission. Additionally, low-smoke fluxing agents cannot be used universally because, for some charges, it is necessary to subsequently add a sprinkling of ammonium chloride. Then there remains the drawback that large quantities of zinc ash and scrapings are developed which must be removed from the surface of the zinc melt and this leads to high zinc losses.